Process and apparatus for making a densified torrefied fuel

ABSTRACT

I claim a process for producing a superior fuel from biomass by first torrefying the biomass and then densifying it to make pellets, cubes or logs with the steps of: heating biomass to a temperature of 150-300 C and preferably to 200-280 C in a heating device, and densification of the resulting torrefied biomass in a device that compresses the hot biomass to make pellets, cubes or logs. The process can be partially or totally self heating by burning the gaseous products of torrefaction in the oven surrounding the torrefication apparatus. The energy required for size reduction after torrefaction is greatly reduced and the energy required for densification is also greatly reduced.

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to the field of energy, and more particularly to a process for producing a superior fuel from biomass (or other energy sources such as peat) by first torrefying the biomass and then densifying it to make pellets, cubes or logs. The process emulates to some extent the processes of Nature in making coal.

[0002] Biomass will be a major source of energy as fossil fuels run out or are seen as environmentally destructive. It has many advantages:

[0003] Renewable

[0004] No global warming effect

[0005] Widely available as low cost lumber or agricultural residues

[0006] but unfortunately also many disadvantages . . .

[0007] Low density, hard to store and ship

[0008] Many sizes and shapes, need to have special handling for each

[0009] Low energy density

[0010] Two processes have been developed in the last three decades which improve the properties of raw biomass.

[0011] Densification into pellets, cubes or logs is a commercial process making useful fuels out of sawdust, paper, plastics and agricultural residues for use in power plants, gasification and home heating. Densification removes many of the disadvantages of biomass and produces a fuel that is

[0012] Much higher density for easy shipping and storing

[0013] Easy to feed in standard equipment

[0014] Uniform and fungible, independent of original shape and size

[0015] Unfortunately it is

[0016] Expensive, using very large machines

[0017] Uses high power

[0018] Has high die wear

[0019] Torrefaction is defined in the Random House Dictionary as “to subject to fire or intense heat, to parch, roast or scorch. When applied to biomass, it is a process consisting of heating biomass to 200-270° C. which

[0020] Increases the mass energy density by first drying, then decarboxylation, from 18 to up to 23 MJ/kg (8000 to 10,000 Btu/lb)

[0021] Renders the fuel waterproof for easy storage and shipment

[0022] Renders the biomass friable, so that particle size reduction is greatly simplified

[0023] Unfortunately it reduces the volume energy density since the product has the same particle size as the starting material with some energy loss.

[0024] Thus, in summary, densification requires high power, has high die wear and requires very heavy machinery. Torrefaction produces a product with low energy density.

PRIOR ART

[0025] Torrefaction

[0026] U.S. Pat. No. 4,954,620 Thermocondensed lignocellulose material, and a method and an oven for obtaining it

[0027] U.S. Pat. No. 4,816,572 Thermocondensed lignocellulose material, and a method and an oven for obtaining it

[0028] U.S. Pat. No. 4,787,917 Method for producing torrefied wood, product obtained thereby, and application to the production of energy

[0029] U.S. Pat. No. 4,553,978 Process for converting ligneous matter of vegetable origin by torrefaction and product obtained thereby

[0030] Densification 4015951 April 1977 Gunnerman 4026678 May 1977 Livingston 4236897 December 1980 Johnston 4308033 December 1981 Gunnerman 4324561 April 1982 Dean et al. 4395265 July 1983 Reilly et al. 4398917 August 1983 Reilly 4494962 January 1985 Christie et al. 4529407 July 1985 Johnston et al. 4561860 December 1985 Gulley et al. 4810255 March 1989 Fay, III et al. 4828573 May 1989 Jelks 4834777 May 1989 Endebrock 5141526 August 1992 Chu 5342418 August 1994 Jesse 5643342 July 1997 Andrews

[0031] It is seen that there are patents on both torrefaction and densification, but I find no combination of densification after torrefaction which is the substance of this patent and which has a number of advantages over the processes taken separately.

SUMMARY OF THE INVENTION

[0032] The primary object of the invention is to provide a superior fuel from biomass. The fuel has up to 30% more energy per kg than biomass and can be up to 3 times as dense as wood or 20% denser than conventional pellets. It can be called a “Biomass Superfuel”.

[0033] Another object of the invention is to provide a fuel higher in energy content than biomass and approaching coal in its fuel value but without the detriments of coal (high ash, sulfur and fossil fuel emissions).

[0034] Another object of the invention is to provide a fuel higher in energy density than any other form of biomass.

[0035] A further object of the invention is to provide a fuel of uniform properties from a variety of forms of biomass.

[0036] Another object of the invention is to provide a fuel that is easier to store and ship than the many forms of biomass which occur naturally.

[0037] Another object of the invention is to provide a fuel that is water resistant.

[0038] Another object of the invention is to provide a fuel that requires less energy to make than conventional densified fuels.

[0039] Another object of the invention is to decrease the energy required for size reduction by initial torrefaction.

[0040] Other objects and advantages of the present invention will become apparent from the following descriptions, taken in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed.

[0041] The drawings constitute a part of this specification and include exemplary embodiments to the invention, which may be embodied in various forms. It is to be understood that in some instances various aspects of the invention may be shown exaggerated or enlarged to facilitate an understanding of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 shows diagrammatically steps in making Torrefied Densified Biomass.

[0043]FIG. 2 shows a typical embodiment of the process.

[0044]FIG. 3 shows an advanced concept in which combustion of a small part of the torrefaction products to provide process heat

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.

[0046] Turning first to FIG. 1 there is shown diagramaticaly the steps in producing densified torrefied biomass, including size reduction and drying of the initial feed if necessary, torrefaction followed by further size reduction if necessary, and densification claimed in this invention, and finally cooling the product to 100 C for bagging or storing. The dark boxes, taken in the order shown, constitute the unique part of this invention.

[0047]FIG. 2 is a typical embodiment of the process, showing biomass feed (1) entering a star valve (2) and being fed to for instance multi hearth heating device (3) driven by a motor (4) and heated by burner (5) with a burner-stack (6), aan exit star valve (7) feeing pellets through a closed feeder (8) to a pelletizer, cuber or logger (9) which produces densified torrefied biomass in the chute (10) and feeds a bag or other storage device (11).

[0048]FIG. 3 shows a modification of FIG. 2 in which the products of torrefaction are burned to produce the heat required for torrefaction. It shows a biomass feed (20) entering a star valve (21) and being fed to for instance an auger heating device (22) driven by a motor, (23) and heated by an oven (24) heated with a fuel gas burner (25) for startup that also can burn the torrefaction products in the oven with a re-burner for burning torrefaction gases (26), an exit stock for the hot combustion gases (27) with an oxygen sensor (28) to maintain the correct air/fuel ratio during heating, a star valve (29) for feeding the finished torrefied biomass to a pelletizer, cuber or logger (30) which feed the densified torrefied biomass to a bag or other storage device (31).

[0049] While the invention has been described in connection with a preferred embodiment in FIGS. 2 and 3, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

[0050] In particular, the means of heating for torrefaction can be the multiple hearth kiln shown in FIG. 2, the heated auger shown in FIG. 3 (also described in U.S. Pat. No. 5,110,785), a fluidized bed heater, a rotary kiln, a batch oven, or any other device capable of raising the temperature to 300° C.

[0051] While a pellet mill is shown in FIGS. 2 and 3 for producing pellets, typically from ¼ to ¾ inch, the process can also be practiced using a cuber to make 1-2 inch cubes or any of several log making machines currently in use to make larger cylinders 2 to 4 inch in diameter.

EXAMPLE

[0052] The following experiment was performed in order to investigate the anticipated improvement in properties and reduction in pressure and work to be expected from densifying torrefied biomass. A quantity of pine sawdust was dried at 100° C. to provide a uniform starting material. The sawdust was loaded into a 1 inch diameter die normally used for making metallurgical specimens and capable of being heated to 300° C. The piston was put in place and the heating begun.

[0053] When the die temperature reached the test temperature the pressure was increased in steps of 1000 psi to 10,000 psi, with recording of the die travel at each pressure. The results of these experiments are shown in Table 1. TABLE 1 Density and HHV of DTB made from pine sawdust at 10,000 PSI and T Wt Work T Density Loss HHV HHV Required¹ Form C g/cm3 % kJ/g kJ/cm3 kWh/tonne Pine Sawdust 20 0.20 0 19.3 5.8 NA Pine pellet 20 1.20 0 19.3 23.2 6.00 Pine pellet 100 1.34 0.7 19.3 25.9 6.09 Pine pellet 150 1.37 2.4 19.6 26.9 6.23 Pine pellet 175 1.42 3.5 19.6 27.9 5.34 Pine pellet 200 1.47 4.4 19.6 28.7 4.45 Pine pellet 225 1.44 8.8 21.4 30.7 3.56 Pine pellet 250 1.32 28.8 23.0 30.3 2.67

[0054] From this it is seen that the high heating value of the pellet is increased from 19.3 to 23 kJ/g by first torrefying at 250° C., an increase of 20%. The density is increased from 1.2 to 1.32 g/cm3, an increase of 10%. 

What is claimed is:
 1. A process for producing a superior fuel from biomass comprising the steps of: Heating the biomass to a temperature of 150-300 C and preferably to 200-280 C; and densification of the resulting torrefied biomass to form pellets, cubes or logs.
 2. A process for producing a superior fuel from biomass comprising the steps of: Drying the biomass, comminuting it to a size compatible with the method of densification, heating the biomass in an auger, fluidized bed or rotary kiln to a temperature of 150-300 C and preferably to 200-280 C; followed by densification of the biomass in a pellet mill, cuber or log maker to make pellets, cubes or logs.
 3. An apparatus to carry out the above process for producing a superior fuel from biomass by first torrefying the biomass in an auger furnace, fluidized bed or rotary kiln and then densifying it using a suitable densification device to make pellets, cubes or logs.
 4. An apparatus to carry out the above process by burning the products of torrefaction in an oven that then heats the torrefaction process, thus reducing or eliminating the need for external fuel. 